Plants and animals have the following distinct properties:
- Plants live from solar energy by photosynthesis, they use solar energy to make sugar and oxygen out of carbon dioxide, which gives them energy. Animals live from the sugar and oxygen plants created and produce carbon dioxide for their energy.
- Animals can move across the planet while plants are tied to the ground.
Clearly, animals have a harder time to survive with no plants within their reach than plants have without animals coming close. This is logical because solar energy is always there while plants are not.
So my question is: Are there animals that can do photosynthesis? It's obvious that an animal with plant-like stateliness would be non-beneficial since it relies on eating other plants for it's energy and there may not always be plants within reach from it's spot.
But animals using the sun and carbon dioxide for energy production does not sound so stupid.
- Night animals could also gather energy in their sleep.
- Much easier than plants, animals could make sure nothing blocks their sunlight.
- Many animals go through periods of hunger because food is scarce, for some of them this period is paired with high sunlight levels. (the dry season f.e.) (EDIT: This is just an idea, of course photosynthesis requires water, which is absent in the dry season. But still, in warm period with enough water, there's sometimes too much animals to feed from the available vegeation.)
Some things I already took into consideration:
- I know that plants, because they are small in mass (compared to the area with which they can collect sunlight) and static, don't need nearly as much energy as animals do. Is this the main reason?
- I know that f.e. reptiles, but in fact all cold-blooded animals, already use the sun's energy. But they only use the heat from the sun to warm their bodies, they don't photosynthesize.
Answer
There are 5 answers, all "yes" (though the first one is disputable).
First: there exists at least one animal which can produce its own chlorophyll:
A green sea slug appears to be part animal, part plant. It's the first critter discovered to produce the plant pigment chlorophyll.
The sea slugs live in salt marshes in New England and Canada. In addition to burglarizing the genes needed to make the green pigment chlorophyll, the slugs also steal tiny cell parts called chloroplasts, which they use to conduct photosynthesis. The chloroplasts use the chlorophyl to convert sunlight into energy, just as plants do, eliminating the need to eat food to gain energy.
The slug in the article appears to be Elysia chlorotica.
Elysia chlorotica is one of the "solar-powered sea slugs", utilizing solar energy via chloroplasts from its algal food. It lives in a subcellular endosymbiotic relationship with chloroplasts of the marine heterokont alga Vaucheria litorea.
UPDATE: As per @Teige's comment, this finding is somewhat disputable.
Second, animals need not produce their own Chlorophyll, and instead symbiotically host organisms that use Photosynthetis - e.g. algae and cyanobacteria. This approach is called Photosynthetic symbioses.
Overall, 27 (49%) of the 55 eukaryotic groups identified by Baldauf (2003) have representatives which possess photosynthetic symbionts or their derivatives, the plastids. These include the three major groups of multicellular eukaryotes: the plants, which are derivatives of the most ancient symbiosis between eukaryotes and cyanobacteria; the fungi, many of which are lichenized with algae or cyanobacteria; and the animals. We, the authors, and probably many readers were taught that animals do not photosynthesize. This statement is true in the sense that the lineage giving rise to animals did not possess plastids, but false in the wider sense: many animals photosynthesize through symbiosis with algae or cyanobacteria.
Please note that while most organisms known for this are fungi, and some rare invertebrates (corals, clams, jellyfish, sponges, sea anemones), there is at least one example of vertebrate like this - spotted salamander (Ambystoma maculatum)
Non-chlorophyll synthesis
A 2010 study by researchers at Tel Aviv University discovered that the Oriental hornet (Vespa orientalis) converts sunlight into electric power using a pigment called xanthopterin. This is the first scientific evidence of a member of the animal kingdom engaging in photosynthesis, according to Wikipedia.
Another discovery from 2010 is possibly a second piece of evidence:
University of Arizona biologists researcher Nancy Moran and Tyler Jarvik discovered that pea aphids can make their own carotenoids, like a plant. “What happened is a fungal gene got into an aphid and was copied,” said Moran in a press release.
Their research article is http://www.sciencemag.org/content/328/5978/624, and they did not consider it conclusive:
The team warns that more research will be needed before we can be sure that aphids truly have photosynthesis-like abilities.
Third, depending on how you understand Photosynthesis, you can include other chemical reactions converting sunlight energy.
If the answer is "usual 6H2O + 6CO2 ----------> C6H12O6+ 6O2 reaction done via chlorophyll", then see answers #1,#2.
But if you simply literally translate the term (synthesizing new molecules using light), then you can ALSO include the process of generating Vitamin D from exposure to sunlight that humans do thanks to cholesterol (link)
Non-biological answer.
As a side bonus, Ophiocordyceps sinensis is referred to as half-animal half-plant (not very scientifically IMHO). But it doesn't do photosynthesis.
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